In wireless mobile communications, active, or phased array, antenna systems are emerging in the market, which are used for beam steering and beam forming applications. Active antenna systems allow increase of network capacity, without increasing the number of cell sites, and are therefore of high economical interest. Such systems comprise a number of individual antenna elements, wherein each individual antenna element transmits RF energy, but adjusted in phase relative to the other elements, so as to create a beam pointing in a desired direction. It is essential for the functionality of the system to be able to measure, control and adjust the phase coherency of the signal being radiated from the various individual antenna elements of the antenna array.
In FIG. 1 a known active antenna system is depicted, formed from several individual transceiver elements 4. A digital baseband unit 6 is coupled to each transceiver element, and each transceiver element comprises a transmit path 8 and a receive path 10. Each path is coupled to an antenna element 12. The transmit path 8 processes a signal from baseband unit 6 and includes a digital to analog converter DAC, a power amplifier PA, and a Diplexer/Filter 15. The receive path 10 processes signals received from antenna element 12, and comprises Diplexer/Filter 15, a low noise amplifier LNA, and an analog to digital converter ADC.
Each transceiver element generates an RF signal which is shifted in phase either electronically or by RF-phase shifters relative to the other transceiver elements. Each antenna element thereby forms a distinctive phase and amplitude profile 14, so that a distinctive beam pattern 16 is formed. It is therefore necessary to align or calibrate all signal phases and amplitudes from the individual transceiver elements at the point where they are transmitted by the antenna elements. To align all transceivers, a common reference is required. The transmitted signal is then compared in phase and amplitude with the reference.
To provide a phase and amplitude reference, two different methods have been used:
1. The signal of one element of the array is used as reference and all other signals are adjusted so that the required coherency to the reference element is achieved. This method usually requires (depending on the size of the array and accuracy) very complex algorithms to mutually adjust the elements, because the adjustment relies on mutual coupling of the elements, which is weak for elements at larger distances. Or a factory-calibration is used, which is complicated to recalibrate if, e.g. during the operation of the array, any phase or amplitude changes in the RF-signal-generation and transmission occurs. This method also requires a dedicated receiver unit, which is able to receive the transmitted signals from the other antenna elements. If receive calibration is also required, a dedicated transmitter is needed for a test signal. The additional receiver and transmitter increase cost and the associated algorithms require extra computational resources.
2. A star-distribution network, wherein a reference is generated in a central unit, which is then distributed to all transceivers, and each transceiver is aligned with the reference. This method is the preferred ones for smaller arrays (number of elements≦10) due to the simpler algorithms required. Critical for the central reference generation calibration method is that the accuracy of the reference distribution is high. Each error in terms of phase or amplitude in the reference will be carried forward to the transmitted/received signal itself. To accurately distribute the phase reference, a centrally generated reference signal is split into a set number of signal paths. Each such path is connected to the respective reference signal input of each transceiver unit of the array by respective transmission lines, the transmission lines being of nominally equal length. This method suffers from three draw backs:
a) Each transmission line has to be of at least half the length of the array size. That means even if an element is located very close to the reference signal generator, it requires a long cable. This increases cost unnecessarily and the volume and weight of the network.
b) The number of transceiver elements is limited to the preset number of signal paths. The network has to be designed for a specific number of elements, which leads to inflexibility.
c) The mechanical accuracy of the transmission line lengths has to be great, that is the tolerances must be small, in view of the requirements for phase and amplitude accuracy of the array itself. For example, for a mobile communication base station antenna with eight to ten elements operating at a frequency of approx 2 GHz, the required phase accuracy is in the order of ±3° among elements. This corresponds to an approximate accuracy of the total line length of ±0.9 mm of a Teflon-filled 50 Ohm-coaxial cable with a total length of approx 700 mm (the array itself is approx 1400 mm long). To ensure this kind of accuracy in a mass production environment is expensive, especially if e.g. thermal expansion during the operation of the antenna and varying bending radii of the different lines within the antenna structure are also taken into account.